Re: [PATCH] crypto: af_alg - Add af_alg_restrict sysctl, defaulting to 1

[Eric Biggers <ebiggers@kernel.org>]

On Tue, Jun 23, 2026 at 12:12:24PM -0700, Andy Lutomirski wrote:
> On Mon, Jun 22, 2026 at 4:49 PM Eric Biggers <ebiggers@kernel.org> wrote:
> >
> > AF_ALG is a frequent source of vulnerabilities and a maintenance
> > nightmare.  It exposes far more functionality to userspace than ever
> > should have been exposed, especially to unprivileged processes.  Recent
> > exploits have targeted kernel internal implementation details like
> > "authencesn" that have zero use case for userspace access.
> >
> > Fortunately, AF_ALG is rarely used in practice, as userspace crypto
> > libraries exist.  And when it is used, only some functionality is known
> > to be used, and many users are known to hold capabilities already.
> > iwd for example requires CAP_NET_ADMIN and has a known algorithm list
> > (https://lore.kernel.org/linux-crypto/bcbbef00-5881-421b-8892-7be6c04b832d@gmail.com/).
> >
> > Thus, let's restrict the set of allowed algorithms by default, depending
> > on the capabilities held.
> >
> > Add a sysctl /proc/sys/crypto/af_alg_restrict with meaning:
> >
> >     0: unrestricted
> >     1: limited functionality
> >     2: completely disabled
> >
> > Set the default value to 1, which enables an algorithm allowlist for
> > unprivileged processes and a slightly longer allowlist for privileged
> > processes.
> 
> In our brave new world of containers, this is a bit awkward.  The
> admin is sort of asking two separate questions:
> 
> 1. Is the actual running distro and its privileged components capable
> of working without AF_ALG or with only the parts marked as being
> unprivileged?
> 
> 2. Is the system running contains that need the unprivileged parts?
> (Which is maybe just sha1 for ip?  I really don't know.)
> 
> Should there maybe be two separate options so that all options are
> available?  Or maybe something between 2 and 3 that means "limited
> functionality and privileged modes are completely disabled"?

If we want to offer more settings we could.  I could see this getting
quite complex pretty quickly once everyone weighs in, though.  There's
quite a bit of value in keeping things simple, even if the offered
settings won't be optimal for every case.

- Eric